An electrochromic material changes its optical properties in response to an electrically driven change in its state of oxidation or reduction. An applied voltage from an external power supply causes electrons to flow to (reduction) or from (oxidation) the electrochromic material. In order to maintain charge neutrality, a charge balancing flow of ions occurs in the electrochromic device. To mediate between the electron and ion flow, an electrochromic device must provide for reversible oxidation and reduction reactions during optical switching. Most prior art devices contain an electrochromic material such as a-WO.sub.3 (a=amorphous) which becomes colored on reduction. There must also be provision in the device for a corresponding oxidation reaction during coloring. The oxidation reaction is usually provided for by the use of a counter electrode.
A major consideration in electrochromic devices is the material of the counter electrode. For variable transmittance electrochromic devices, the oxidation and reduction of the counter electrode must not interfere with the transmittance modulation of the device. Counter electrodes in variable transmittance devices can utilize materials that are substantially transparent and which undergo very little optical modulation on reduction and oxidation. Examples of such materials are Nb.sub.2 O.sub.5 and TiO.sub.2 (S. Cogan et al., Proc. S.P.I.E., vol 562, (1985), pp. 23-31) or a "macroporous" layer of crystalline WO.sub.3 (U.S. Pat. No. 4,278,329, K. Matsuhiro and Y. Masuda, July 1981). The counter electrode may also be a reduction-oxidation couple dissolved in a liquid or semi-solid electrolyte (U.S. Pat. No. 4,550,982, Y. Hirai, November 1985). The most useful counter electrode, however, is itself an electrochromic material which colors and bleaches in tandem with the principal electrochromic material. In the case of electrochromic devices employing a combination of a-WO.sub.3 and IrO.sub.2, for example, the a-WO.sub.3 colors on reduction and the IrO.sub.2 colors on oxidation. Both layers therefore contribute to the optical modulation, improving both the efficiency of the optical change and increasing the maximum transmittance range over which the device may switch. The a-WO.sub.3 /IrO.sub.2 electrochromic device employs H.sup.+ as the charge-balancing counter ion. Examples of such devices in prior art include Takahashi et al., U.S. Pat. No. 4,350,414, September 1982 and Cogan et al., Proc. S.P.I.E., vol 823, (1987) pp. 106-112.
In many applications, electrochromic devices will encounter elevated temperatures and high solar irradiance. Temperatures of automobile sunroofs, aircraft canopies, and building windows may exceed 100.degree. C. Any deleterious effect of such high temperatures is compounded by the concomitant high levels of irradiance which may result in photothermal or photoelectrochemical degradation. A major contributor to such degradation is residual water in the electrochromic electrodes or in the ion conducting layer. It is common in many prior art electrochromic devices to employ a proton (H.sup.+) or hydroxyl (OH.sup.-) as the counter ion. The use of these ions necessitates the incorporation of some amount of lattice water in both the ion conducting and electrochromic layers. The H.sub.2 O is necessary to achieve the desired level of ionic conductivity. It is most desirable to avoid residual water by employing a counter ion other than H.sup.+ or OH.sup.-.
If an electrochromic device is to have reproducible and predictable optical switching behavior and exhibit a high switching cycle lifetime, then the electrochemical oxidation and reduction reactions at both the electrochromic electrode and the counter electrode must be reversible and free of parasitic side reactions. These considerations are of paramount importance in applications where high switching cycle lifetimes and long service life are required such as a variable transmittance glazing on architectural glass.
Furthermore, if the electrochemical reactions at both electrodes are well-defined and reversible, an electrochromic device may be constructed with optical properties uniquely determined by the switching voltage. A unique relationship between optical properties and applied switching voltage is advantageous because it eliminates the need for external sensors to measure the optical state during switching and it greatly simplifies the design of power supplies and makes it practical to switch a large number of electrochromic devices in an identical manner (such as electrochromic windows in an office building).